1. Field of the Invention
The present invention relates to an image forming apparatus, such as a color copier, a color printer, a color multifunctional device, and the like, employing an intermediate transfer system or a direct transfer system including a mechanism to switch between monochromatic and multicolor modes.
2. Description of the Background Art
There exists a color image forming apparatus, such as a copier, a printer, an image forming apparatus, and the like, for forming a multi-color (i.e., color) image using a circulating belt, such as an intermediate transfer belt, a photoconductor belt, a sheet transfer belt, and the like. As this type of a color image forming apparatus, there is a tandem color image forming apparatus, for example, separately incorporating image formation units forming toner images of yellow, magenta, cyan, black, and a special color, such as gloss, and the like, respectively, on a belt, such as an intermediate transfer belt, and the like., stretched around multiple rollers.
The tandem color image forming apparatus forms the different color toner images on photoconductors in the respective image formation units, and sequentially transfers and superimposes those onto an intermediate transfer belt or a recording medium (i.e., sheet) transported by the sheet transfer belt. When the tandem color image forming apparatus employs an intermediate transfer system, a toner image on the intermediate transfer belt is secondarily transferred onto a recording medium, such as a sheet, and the like. In either situation, the recording medium, onto which the toner image is ultimately transferred, passes through a fixing device that fixes the image on the recording medium. Then, the recording medium is discharged from the apparatus.
In general, such an image forming apparatus is configured to switch between a multicolor mode to form a color image and a monochromatic mode to form a monochromatic image (generally a grayscale image). Accordingly, a prescribed mechanism is provided to reposition the belt such that the belt engages all photoconductors in the multiple image formation units in the multicolor mode, and only engages one of the photoconductors of the multiple image formation units in the monochromatic mode.
Generally, in a belt driving system in which a circulating belt is supported by multiple rollers and is circulated by one of the multiple rollers serving as a driving roller, belt displacement of the circulating belt (so-called belt walk) in a lateral, widthwise direction (i.e., a direction perpendicular to a belt running direction) may occur. In the above-described tandem color image forming apparatus, belt walk induces relative displacement of each color image when the color images are transferred and superimposed, e.g., on the intermediate transfer belt, thereby causing color displacement or color unevenness, and the like. Therefore, to obtain output of high-quality images, belt walk needs to be appropriately corrected.
It is known that when a position of a roller stretching the belt is changed, a displacement speed varies accordingly and takes a certain amount of time until it is stabilized. Therefore, when switching between the monochromatic mode and the full-color mode and thus repositioning the belt, such as an intermediate transfer belt, and the like, the belt walk speed varies. As a result, a transfer position of each color-toner image deviates in the main scanning direction (i.e., a direction orthogonal to the belt, or the belt widthwise direction) due to belt walk when a prescribed mode is switched to a full-color mode, and accordingly, an abnormal image with color displacement is formed.
Several techniques have been proposed to correct belt walk. For example, a belt walk control system (hereinafter, simply referred to as a steering system) is known in which one of the multiple rollers supporting a circulating belt serves as a correction roller (hereinafter, simply referred to as a steering roller) and is slightly inclined in a direction not to change a perimeter of the belt so as to control the displacement thereof. There is also a system (hereinafter, simply referred to as a displacement stopping system) that forcibly minimizes the belt walk with a rib or a guide or the like. The above-described steering system is generally more reliable than the displacement stopping system because a force applied to the belt is weaker.
However, a displacement speed varies also in the steering system for a certain amount of time when a primary transfer roller initially either contacts or separates from the belt, and consequently a color displacement may occur when a multicolor image is outputted during that time period. Further, when image formation modes are switched in the steering system, a steep change occurs in displacement direction. Furthermore, the stability of the steering roller varies with the positioning of the belt.
For this reason, to avoid such color displacement and an abnormal image caused by the belt walk in the main scanning direction when switching from a monochromatic mode to a full-color mode in an image forming apparatus, a displacement amount of the belt, such as intermediate transfer belt, and the like, in a widthwise direction is detected and color image formation is not executed until a detected value falls below a prescribed threshold level, as described in Japanese Patent Application Publication No. 2009-63909 (JP-2009-63909-A), for example.
However, allowing time for the belt to stabilize after switching from the monochromatic mode to the full-color mode in this way degrades, productivity, i.e., the efficiency of image formation that is especially highly valued in the market.